1. Field of the Invention
This invention relates to a shaft seal for materials processing machinery and, more specifically, to a pump or impeller type seal that is preferably fluid purged for use in attrition mills and the like, such as an atmospheric wood pulp refiner.
2. Description of the Prior Art
In the prior art and, in particular, in connection with atmospheric wood pulp refiners, many types of rotor shaft seals have been employed to inhibit ingress of the material being processed into the area or zone where the main shaft of the refiner passes through its surrounding, closely fitting, machine housing. Among main objects of such seals are the desiderata to prevent abrasion or erosion of the shaft and/or housing at such zones by the natural or inherent abrasive property of certain processed materials, as well as to prevent such materials from damaging shaft bearings or journals that are frequently located adjacent to the juncture of the main shaft with the machine housing.
In one of the best of the known prior art refiner seals, as employed by applicant's assignee, a mechanical seal is used in conjunction with a lip seal and a cone shaped, centrifugal type, fluid impeller. This system provides a very effective seal that is particularly useful for pressurized refiners. However, the mechanical seals are very costly and the lip seals require replacement periodically. Changing a lip seal necessitates shutting down the machine for 10 to 12 hours. Moreover, optimum operation of the cone shaped, centrifugal type, fluid impeller can be adversely affected by excessive movement of the main shaft due to thermal expansion and end play. Also, the cone shaped, centrifugal type impeller, notwithstanding fluid injection, is found to be inherently susceptible to permitting some processed materials to pass thereover or therepast and cause abrasive damage to the main shaft and/or surrounding housing, as well as adjacent main shaft bearing structures. In addition, while pressurized refiners require an exotic system of seals to maintain desired pressure during refining, such is not the case with atmospheric refiners due to their reduced operating pressure.